This invention relates to a structure for and process of manufacture of a semiconductor device, in particular, a power MOSFET or IGBT, and more particularly relates to a novel back electrode for the back or bottom electrode of such devices.
Power MOSgated products such as power MOSFETs, IGBTs and power integrated circuits in which intelligence circuits are integrated into such devices are well known. In the process of manufacture of such devices, the back or bottom surface of the wafer, from which die are subsequently cut, is commonly ground by a grinding tool before the back contact, usually a three-metal layer of Cr Ni Ag or Ti Ni Ag is applied thereto. While this grinding operation improves metal adherence to the back surface, the grinding operation frequently causes wafer breakage. Wafer breakage at this point in the processing is particularly expensive because almost the total cost of processing has already been invested in the wafer.
Prior art back metals commonly employ chromium and titanium as the first metal, primarily for their ability to form a coherent silicon dioxide/chromium oxide or silicon dioxide/titanium dioxide bond. This first metal is commonly described as the glue layer, as its function is to adhere the silicon to the nickel, which is the second metal of the trimetal. Conduction takes place via tunneling as the oxides are extremely thin. Generally, P type silicon presents a greater problem in establishing an ohmic contact than N type and the higher the resistivity of the P or N type silicon, the worse the problem becomes.